1. Field
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for the forward link power control in a communication system.
2. Background
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation techniques of CDMA have significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” assigned to the assignee of the present invention and is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” also assigned to the assignee of the present invention and is incorporated by reference herein. Furthermore, the CDMA system can be designed to conform to the “TIA/EIA/IS-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” hereinafter referred to as the IS-95-A standard or TIA/EIA/IS-95-A.
CDMA, by its inherent nature of being a wideband signal, offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links to a mobile user or remote station through two or more base stations. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing signals arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled “METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,” and U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention and incorporated by reference herein.
The reverse link refers to a transmission from a remote station to a base station. On the reverse link, each transmitting remote station acts as an interference to other remote stations in the network. Therefore, the reverse link capacity is limited by the total interference due to transmissions from other remote stations. The CDMA system increases the reverse link capacity by transmitting fewer bits, thereby using less power and reducing interference, when the user is not speaking.
To minimize interference and maximize the reverse link capacity, the transmit power of each remote station is controlled by three reverse link power control loops. The first power control loop adjusts the transmit power of the remote station by setting the transmit power inversely proportional to the received power on the forward link. In an IS-95-A system, the transmit power is given by pout=−73−pin where pin is the power received by the remote station given in dBm, pout is the transmit power of the remote station given in dBm, and −73 is a constant. This power control loop is often called the open loop.
The second power control loop adjusts the transmission power of the remote station such that the signal quality, as measured by the energy-per-bit-to-noise-plus-interference ratio Eb/Io, of the reverse link signal received at the base station is maintained at a predetermined level. This level is referred to as the Eb/Io set point. The base station measures the Eb/Io of the reverse link signal received at the base station and transmits a reverse link power control bit to the remote station on the forward traffic channel in response to the measured Eb/Io. The reverse power control bits are set 16 times per 20 msec frame, or at an 800 bps rate. The forward traffic channel carries the reverse link power control bits along with the data from the base station to the remote station. This second loop is often called the inner closed loop.
The CDMA communication system typically transmits packets of data as discrete data frames. Thus, the desired level of performance is typically measured by the frame-error-rate (FER). The third power control loop adjusts the Eb/Io set point such that the desired level of performance, as measured by the FER, is maintained. The required Eb/Io to obtain a given FER depends upon the propagation conditions. This third loop is often called the outer closed loop. The power control mechanism for the reverse link is disclosed in detail in U.S. Pat. No. 5,056,109, entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM,” assigned to the assignee of the present invention and incorporated by reference herein.
The forward link refers to a transmission from a base station to a remote station. On the forward link, the transmission power of the base station is controlled for several reasons. A high transmission power from the base station can cause excessive interference with the signals received at other remote stations. Alternatively, if the transmission power of the base station is too low, the remote station can receive erroneous data transmissions. Terrestrial channel fading and other known factors can affect the quality of the forward link signal as received by the remote station. As a result, each base station attempts to adjust its transmission power to maintain the desired level of performance at the remote station.
Power control on the forward link is especially important for data transmissions. Data transmission is typically asymmetric with the amount of data transmitted on the forward link being greater than on the reverse link. With an effective power control mechanism on the forward link, wherein the transmission power is controlled to maintain the desired level of performance, the overall forward link capacity can be improved.
A method and apparatus for controlling the forward link transmission power is disclosed in U.S. Pat. No. 6,035,209, entitled “METHOD AND APPARATUS FOR PERFORMING FAST FORWARD POWER CONTROL IN A MOBILE COMMUNICATION SYSTEM,” hereinafter the '209 patent, filed Mar. 31, 1995, assigned to the assignee of the present invention and incorporated by reference herein. In the method disclosed in the '209 patent, the remote station transmits an error-indicator-bit (EIB) message to the base station when a transmitted frame of data is received in error. The EIB can be either a bit contained in the reverse traffic channel frame or a separate message sent on the reverse traffic channel. In response to the EIB message, the base station increases its transmission power to the remote station.
One of the disadvantages of this method is the long response time. The processing delay encompasses the time interval from the time the base station transmits the frame with inadequate power to the time the base station adjusts its transmission power in response to the error message from the remote station. This processing delay includes the time it takes for (1) the base station to transmit the data frame with inadequate power, (2) the remote station to receive the data frame, (3) the remote station to detect the frame error (e.g. a frame erasure), (4) the remote station to transmit the error message to the base station, and (5) the base station to receive the error message and appropriately adjust its transmission power. The forward traffic channel frame must be received, demodulated, and decoded before the EIB message is generated. Then the reverse traffic channel frame carrying the EIB message must be generated, encoded, transmitted, decoded, and processed before the bit can be used to adjust the transmit power of the forward traffic channel.
Typically, the desired level of performance is one percent FER. Therefore, on the average, the remote station transmits one error message indicative of a frame error every 100 frames. In accordance with the IS-95-A standard, each frame is 20 msec long. This type of EIB based power control works well to adjust the forward link transmit power to handle shadowing conditions, but due to its slow speed is ineffective in fading except in the slowest fading conditions.
A second method for controlling the forward link transmission power utilizes the Eb/Io of the received signal at the remote station. Since the FER is dependent on the Eb/Io of the received signal, a power control mechanism can be designed to maintain the Eb/Io at the desired level. This design encounters difficulty if data is transmitted on the forward link at variable rates. On the forward link, the transmission power is adjusted depending on the data rate of the data frame. At lower data rates, each data bit is transmitted over a longer time period by repeating the modulation symbol as described in TIA/EIA/IS-95-A. The energy-per-bit Eb is the accumulation of the received power over one bit time period and is obtained by accumulating the energy in each modulation symbol. For an equivalent amount of Eb, each data bit can be transmitted at proportionally less transmission power at the lower data rates. Typically, the remote station does not know the transmission rate a priori and cannot compute the received energy-per-bit Eb until the entire data frame has been demodulated, decoded, and the data rate of the data frame determined. Thus, the delay of this method is as described in the aforementioned U.S. Pat. No. 6,035,209, and the rate is one power control message per frame. This is in contrast with the reverse link approach in which there can be one power control message (bit) sixteen times per frame as in TIA/EIA/IS-95-A.
Other methods and apparatus for performing fast forward link power control are described in the aforementioned U.S. Pat. No. 6,035,209, U.S. Pat. No. 6,137,840, entitled “METHOD AND APPARATUS FOR PERFORMING FAST FORWARD POWER CONTROL IN A MOBILE COMMUNICATION SYSTEM,” filed Nov. 15, 1995, issued Oct. 24, 2000, to Edward G. Tiedemann Jr., et al., U.S. Pat. No. 5,903,554, entitled “METHOD AND APPARATUS FOR MEASURING LINK QUALITY IN A SPREAD SPECTRUM COMMUNICATION SYSTEM,” filed Sep. 27, 1996, U.S. Pat. No. 5,893,035, entitled “CENTRALIZED FORWARD LINK POWER CONTROL,” filed Sep. 16, 1996, and U.S. Pat. No. 6,075,974, entitled “METHOD AND APPARATUS FOR ADJUSTING THRESHOLDS AND MEASUREMENTS OF RECEIVED SIGNALS BY ANTICIPATING POWER CONTROL COMMANDS YET TO BE EXECUTED,” filed Nov. 20, 1996, all are assigned to the assignee of the present invention and incorporated by reference herein.
The fundamental difference between the forward link and the reverse link is that the transmission rate does not need to be known on the reverse link. As described in the aforementioned U.S. Pat. No. 5,056,109, at lower rates, the remote station does not transmit continuously. When the remote station is transmitting, the remote station transmits at the same power level and the same waveform structure regardless of the transmission rate. The base station determines the value of a power control bit and sends this bit to the remote station 16 times per frame. Since the remote station knows the transmission rate, the remote station can ignore power control bits corresponding to times when it was not transmitting. This permits fast reverse link power control. However, the effective power control rate varies with the transmission rate. For TIA/EIA/IS-95-A, the rate is 800 bps for full rate frames and 100 bps for ⅛ rate frames.
An alternative reverse link architecture is described in the U.S. Pat. No. 5,930,230, entitled “HIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEM,” hereinafter the '230 patent, filed May, 28, 1996, assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the '230 patent, an auxiliary pilot is introduced into the reverse link. The pilot level is independent of the transmission rate on the reverse link. This permits the base station to measure the pilot level and to send the reverse link power control bit to the remote station at a constant rate.